A hip stem is a prosthetic medical implant that replaces the upper portion of a femur that has been resectioned during an arthroplasty procedure. The hip stem includes an elongate portion and a rounded head. The elongate portion is insertable into the medullary canal of the resectioned femur, whereas the rounded head extends from the femur for mating with an acetabular cup that is securable to a hip socket of the pelvis. Together, the hip stem and acetabular cup comprise a prosthetic hip joint.
During an arthroplasty procedure bone cement such as polymethymethacrylate (PMMA) can be deposited into the medullary canal of the femur and the elongate portion of the hip stem is inserted into, and enveloped by, the mass of bone cement. Rough or textured portions of the hip stem bond securely with the bone cement to provide a first interface, and the cement infuses into and bonds with the bone at a second interface. Thus, the hip stem is rigidly secured in place within the femur. However, it has been discovered that even if the femoral stem is flexible, repeated loading and unloading of the prosthetic hip joint during physical activity can cause the bone cement to fatigue and fail.
Uncontrolled breakdown or failure of the bone cement has undesirable consequences, such as free particle generation that can lead to osteolysis or "cement disease" of host bone tissue. Additionally, loose bone cement particles can migrate from the medullary canal to articulating surfaces and act as third body wear generators. This can cause excessive wear of the femoral head and/or a metallic or polymeric counterface within the acetabular cup. With respect to polymeric counterfaces, cement particulate can become embedded in the polymer surface and act as a second body wear surface against the metallic articulating surface of the femoral head.
As the cement breaks down, the hip stem increasingly loosens. And as the hip stem loosens, the bonds between the bone and the cement, and the stem and the cement weaken. The breakdown of these bonds allows the stem to subside or sink into the medullary canal, thereby shortening the leg. As the leg becomes shorter, an undesirable change in patient gait can develop which can continue to deteriorate as the stem subsides. Eventually, the gait deteriorates to such a degree that surgical revision (replacement) of the stem is indicated. Avoidance of the above-described sequence of events would be highly desirable.